Discontinuous fiber-reinforced ceramic matrix composite (CMC) is a useful material for many applications such as automobile engines and exhaust systems. For example, discontinuous fiber-reinforced CMC is used in components such as pistons for automobile engines as well as catalytic converters, exhaust manifolds, brake rotors and brake pads. These fiber-reinforced CMC components are resistant to extremely high temperatures and exhibit surprising strength and durability.
Discontinuous fiber-reinforced CMC is typically prepared by cutting or chopping the fiber into a predetermined length while the fiber is dry. The dry, discontinuous fiber is then mixed with a high-viscosity resin or resin and fillers to form a mixture. This mixture is then used to fabricate a discontinuous fiber-reinforced CMC component.
One problem associated with this fabrication method is that when the dry fiber is cut or chopped it tends to break up the tow or yarn into smaller diameter filament bundles than the original continuous fiber material. Moreover, when the fiber is chopped into less than approximately one-inch lengths, the fibers tend to become unraveled and frayed. In fact, these chopped fibers have "fluffy" consistency and resemble cotton. This unraveling and fraying of the fibers adversely affects the strength and durability of the final CMC component.
Another problem with this fabrication method is that the unraveled and frayed discontinuous fiber is extremely difficult to mix with high-viscosity resins and resins/fillers. Specifically, the fluffy consistency of the chopped fibers causes them to coagulate when mixed with the resin. Consequently, the mixture is not a homogeneous mixture but has fibers and possibly fillers distributed unevenly throughout the mixture. In addition, the mixture typically contains unwanted areas of unmixed dry fibers. Once again, this problem adversely affects the strength and durability of the final CMC component. Moreover, ease of manufacturing is negatively impacted because the mixture cannot easily be poured or injected due to its nonuniformity.
A further problem with the fabrication method relates to dust generation. In particular, the unraveling and fraying of the chopped fibers creates a great deal of dust. This dust generation makes the discontinuous fibers difficult to handle and can also clog machinery and other devices. In addition, excessive dust generation creates health hazards for humans and increases manufacturing costs because of the need to install safety equipment.
Therefore, what is needed is a fiber that will not unravel or fray when cut or chopped. This would thereby improve the strength and durability of CMC components. What is also needed is a fiber that mixes well with high-viscosity resins and resins/fillers. Specifically, the fiber would not coagulate when mixed with the resin and would create a more homogeneous mixture. This would simplify manufacturing by allowing preparation of a uniform mixture that could easily be poured, injected and compression molded.
What is further needed is a discontinuous fiber that avoids dust generation. This would permit easier handling of the discontinuous fibers. Moreover, this would decrease the hazards to both machinery and human health.
Whatever the merits of the above-mentioned methods of fiber preparation for discontinuous fiber-reinforced CMC, they do not achieve the benefits of the present invention.